He's Got the Power
In the most practical terms possible, you want to be 100 percent sure that your wedding officiant has what it takes to make your nuptials run smoothly. During your pre-wedding interview, you should consider asking him a few standard questions to get a feel for his personal style and organizational skills. For example, how long has he been officiating these blessed events? Everyone has to start somewhere, but I'd be hesitant to plop a first-timer at the end of the aisle for fear that he'd forget his lines or even the names of the happy couple. Also, since you're a reasonable gal and you know that emergencies crop up from time to time, it wouldn't hurt to find out how he'd handle a situation that would require him to bail at the last minute. And if your officiant is signing a marriage certificate, make sure he's been ordained with the power to do so.
Attend your meetings armed with these and other smart questions. Bear in mind that it's vitally important for your personalities to be complemented by the officiant, so if he's the kind to pop too many jokes while you're a by-the-script kind of bride, you should continue the interviews until you find the perfect match.